Afterburner will keep you from going excessively far, but 1.1 - 1.13v is usually a good place to stop unless you are running a waterloop through it. If you want to be extra safe, 1.09v is a good place to stop... Temperatures usually start to hold back the overclocks though before dangerous voltages get used, and the GTS 450 has good enough VRM's to handle the voltage.

You can get some serious overclock power at those voltages. Edited by Talon95 - 5/20/11 at 9:13pm

Most GTS450s are capable of overclocks in the mid 900mhz range (core). Some can surpass 1000mhz. A voltage bump will be required. How high you go is dictated by the overclocking software's preset voltage limit (unless you decide to surpass it) and the temperature/noise level you hope to achieve. If temperatures and noise are within desired levels, you should be fine pumping the card with up to 1.162v. GF106-based GPUs run cool (and the reference design is fairly capable) - You will generally be limited by the GPU itself rather than GPU temperature when overclocking a GTS450.